The present invention relates generally to plasma chamber tuners, and in particular to a solid-state tuner used to match the impedance of an RF generator to a plasma chamber or similar non-linear load.
A plasma chamber is a low-pressure, gas filled enclosure used in processing operations such as RF sputtering, plasma depositions, and reactive ion etching. These processing operations are used primarily for integrated circuit and compact disc fabrication. For those processing operations requiring an RF power signal, the plasma chamber is excited by an RF generator that is generally operating in the ISM frequency bands of 13.56 MHz, 27.12 MHz, and 40.68 MHz. The input impedance of the plasma chamber is highly non-linear, varying during the different operating modes of the chamber. Although lower and higher frequencies can be employed with a plasma processing system, 13.56 MHz is the predominant frequency used in the RF generator market.
Typically, an RF generator produces an RF wave at a predetermined frequency such as 13.56 MHz. The generator is coupled to the plasma chamber through a power conduit. The output of the RF generator is typically designed to be a fixed, known impedance such as 50 ohms. Since there is typically a severe impedance mismatch between the RF generator and the plasma chamber, an automatic impedance matching tuner is generally coupled between the generator and the chamber.
Impedance matching tuners have evolved recently from electromechanical tuners to solid-state tuners. Electromechanical tuners are typically motor driven devices that control a variable capacitor to match the output impedance of the generator to the plasma chamber. Although, electromechanical tuners are capable of matching an RF generator to a wide range of plasma chamber input impedances, they possess many inherent deficiencies including slow response, poor reliability, high and unpredictable power loss, and high cost.
In response to the aforementioned deficiencies of electromechanical tuners, a solid-state tuner was recently developed as described in U.S. Pat. No. 5,473,291 which is hereby incorporated by reference in its entirety. The advent of the solid-state tuner rectified many of the listed deficiencies of the electromechanical tuners providing faster response and increased reliability at low power levels, a predictable power loss, and lower cost. However, conventional solid-state tuners are limited in the RF power level that may be applied while the impedance is adjusted. Adjusting the impedance of the tuner while RF power is applied is referred to as hot switching. Conventional solid-state tuners universally exhibit poor reliability when hot switching is attempted at RF power levels exceeding approximately 300 watts.
The present hot switching method and impedance matching circuit extends the tuning range of matching circuits to include increased power levels. The hot switching method and circuit includes coupling a controlled impedance network between an RF generator output and a plasma chamber input for matching impedances. The controlled impedance network includes an RF switch for switching a predetermined impedance. A device performance characteristic of the RF switch is determined. RF power is applied from the RF generator through the controlled impedance network to the plasma chamber. A signal characteristic of the impedance match is measured. The RF switch is controlled based upon the measured signal characteristic such that the impedance match is driven towards a predetermined matching range. The RF switch is switched any speed based upon the device performance characteristic.